Stimulating investment in pearl farming in ... - World Fish Center

Abundance, Size Structure and Quality of Silver-lip Pearl Oysters in the Solomon Islands Final, June 20084.0 DISCUSSIONThe current surveys undertaken by divers indicated that silver-lip pearl oysters werepresent at six of the eight historical fishing areas sampled and that densities at Waghina, Kiaand Mboli Passage (Narula Passage) were considerably greater than those at Malaita, MarauSound and Sandfly Passage. However, the maximum average density within any onelocation was less than 1.25 animals per 400 m 2 and none of these stocks could be consideredcommercially significant. The highest density recorded per 400 m 2 transect was 12.4 andoccurred on Transect 1, Site 2, Mboli Passage (Appendix 1). The remote video censusesindicated that pearl oysters may also be present in deeper water at Kia, Waghina, Malaitaand Sandfly Passage, however, even here the “possible” pearl oysters were present at lowdensities. The low densities of silver-lip oysters observed are consistent with previoussurveys undertaken in Kia and Waghina in the late 1980’s and early 1990’s (Colgan, 1993).While those surveys used different sites and different methods and are not strictlycomparable, they recovered approximately 50 shells in a week of diving. The current surveysuggests that stocks have either recovered little since fishing was prohibited in 1994 or thatthey did recover, but have subsequently been depleted by natural mortality or illegalfishing.There is certainly evidence that natural mortality can reduce silver-lip pearl oyster stocks,with Ngella being decimated by disease in the early 1970s (Gauld 1975 in Colgan 1993) andreports of a “red tide” causing heavy mortality of silver-lip oysters at Kia in 1971 (Colgan1993), but it is not known whether these have caused large-scale natural mortality in recentyears. No anecdotal evidence of disease was offered in interviews with local villagers, whodid however confirm that fishing of pearl oysters has taken place at both Waghina and KiaPassage since exploitation was prohibited. Illegal exports of silver-lip and black-lip pearloysters from these locations have been intercepted by Fisheries, however, it is not knownhow much shell was collected (J. Leqata pers. comm.).The population structure that we observed was biased towards large individuals, with anoverall median size of 210 mm. Pearl farmers in Western Australia prefer oysters aroundthe 125 mm size class to those in the 150-160 mm size classes for round pearl production(Fletcher et al. 2006) and will not harvest pearl oysters larger than 175 mm DVM. We foundno shell smaller than 160 mm and overall only 4.2% were less than 175 mm DVM. Themajority of oysters found during the current survey were beyond the limit currentlyconsidered most suitable for pearl farming. Previous surveys had also indicated that mostof the oysters found were >200 mm in DVM and too large for pearl culture (Nichols 1988and Batty and Kile 1990 in Colgan 1993).The simplest explanation for the presence of small populations of large individuals, in thecurrent survey is a persistent failure of reproduction, spat settlement and/or recruitmentover the past decade. The reproductive success of many marine invertebrates is episodic,with a low background level of recruitment occasionally being interrupted by very goodyears (Gaines et al. 1985; Dye 1990; McShane and Smith 1991; Hughes and Tanner 2000).The size frequency distributions of some of the fished silver-lip oyster populations inAustralia suggest that recruitment is also episodic in some populations of pearl oysters (Hartand Friedman 2004). In broadcast spawners, the distribution and abundance of adults has aprofound influence on individual reproductive success (Levitan et al. 1992). The reductionin abundance and greater spacing of adult oysters brought about by exploitation is a factorthat may have contributed to poor reproductive success and low larval production. In someThe Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 12